The invention described herein pertains generally to a method for processing plastic tubing ends, particularly those of polyethylene, and more particularly, medium density polyethylene. The method refers generally to belling and reforming tubing ends without compromising wall thickness, thereby enabling the work piece to be suitable for plumbing applications. In one application of this invention, the technology will involve crosslinking to at least a limited degree, i.e., less than 10%, preferably less than 5%, and more preferably, less than 1%. In an alternative embodiment, the technology will eliminate this crosslinking step and result in a tubing end which is more flexible than polypropylene or crosslinked polyethylene, yet which will be suitable for use in low pressure, cold water side applications, and especially adapted for use in refrigeration applications, e.g., ice maker cold water input lines.
In plumbing installations, copper tubing is widely employed. In risers, used for connecting tubing to fixtures or tanks, the end of the copper tubing is shaped to form a bulb sealing surface and such bulb includes a shoulder permitting the tubing and thus the bulb sealing surface to be drawn into biting or sealing engagement with the fixture. The cost of such copper tubing and the cost of forming the same to permit the connection to such fixtures or tanks is substantial.
More recently, polybutylene was approved for use in plumbing. Tubing or pipe made of polybutylene is normally joined by heat-fusion techniques, by mechanical compression, and by cold flaring. In order to provide such polybutylene tubing with a bulb sealing surface or an end cap for such purposes, a variety of techniques have been employed. Two commonly employed techniques are: (1) spin welding a separately molded bulb onto the outer diameter (O.D.) of the end of a tube; or (2) insert molding a bulb onto the O.D. of the end of a tube. All such processes have cost and performance drawbacks. Most require separately molded parts which must be joined to the tubing in assembly operations. Moreover, a two-part tubing end cap or bulb sealing construction does not have the performance integrity or the expected useful life of the tubing itself. In the spin welding technique, excessive clamping pressures may cause the loaded part to become dislodged or separated from the O.D. of the tubing and the interface of the parts provides a possibility of leakage. In the case of a neoprene or like washer employed on the O.D. of the tubing, the same interface leakage susceptibility is present. Moreover, a flange formed to receive the washer may itself create a point of weakness if excessive clamping pressures are employed. Further neoprene washers are known to deteriorate with age and temperature exposure. Lastly, insert molding forces hot material over a cold tube surface, which can separate from the tube.
The solution to this problem of providing polybutylene tubing with an attached bulb sealing surface of unitary construction is detailed in U.S. Pat. Nos. 4,316,870, 4,446,084, and 4,525,136. The thrust of these references however is to teach the ability to maintain a constant diameter opening within the tubing, while the wall thickness is variable. This is of necessity, due to the configuration of the mold cavity, and insertion of the mandril inside the tubing during the processing steps.
A corresponding associated problem with the formation of the above-described male end of the polybutylene tubing, is the ability to bell an opposed end of the tubing, without any accompanying wall thickness compromise, which would make the product unsuitable for all plumbing applications, for which polybutylene has been approved, provided that a wall thickness can be maintained at 0.062xe2x80x3xc2x10.010xe2x80x3, as defined by ASTM 3309. In particular, it is desirable to use xe2x85x9cxe2x80x3 O.D. polybutylene tubing with wall thickness of {fraction (1/16)}xe2x80x3 (0.062xe2x80x3) and subsequently insert a xc2xdxe2x80x3 CTS (copper tube size) fitting of nominal 0.501xe2x80x3 O.D. The only way this can be achieved is through belling one end of the tubing from xe2x85x9cxe2x80x3 O.D. (xc2xcxe2x80x3 I.D.) to ⅝xe2x80x3 O.D. (xc2xdxe2x80x3 I.D.). While it is possible to use ⅝xe2x80x3 O.D. tubing to start, this uses more raw materials than necessary.
Prior art solutions to the formation of a bell on one end of polybutylene tubing is by heating a portion of the end of the tubing, followed by insertion of a mandril into the heated open end, the O.D. of the mandril being matched to the targeted inner diameter of the tubing. While this approach will bell the tubing, it is incapable of reproducibly making tubing products with a constant wall thickness of 0.062xe2x80x3xc2x10.010xe2x80x3 throughout the belled end, particularly in the neck region of the bell. This is due to the fact that the bell is made by expanding the I.D. and thus thinning the walls. A solution to this problem is found in U.S. Pat. No. 5,527,503.
More recently, the trend has been to shift from thermoplastic materials, e.g., polypropylene, polybutylene, etc. to thermoset materials, e.g., crosslinked polyethylene. However, this shift in materials is not simple in that there are several processing changes which must be incorporated in order to fabricate acceptable parts. Since thermosets cannot be extruded like thermoplastics, different processing conditions must be employed in different sequences in order to achieve similar functionality for the product. For example, it is not possible to simply take a crosslinked polyethylene tube and mold it into a bulb end by taking the polybutylene technology taught in the prior art. Previously crosslinked material will not chemically bond to itself even when heated to the clear state. This means that the material in the formed ends is not completely sealed upon itself, but rather molded in place with pressure. One prior art solution to this problem is the use of metal inserts which are positioned into crosslinked polyethylene tubes and subsequently crimped in order to achieve a fitting. This is an inherent weak spot in the final product, and the industry has long sought to find a solution to the problem of developing a one-piece plumbing part made out of a thermoset plastic. Solutions to this problem can be found in U.S. Pat. Nos. 5,622,670, 5,833,279 and 6,070,916.
Most recently, with cost playing an increasing role in component fabrication matters, another shift is occurring which capitalizes on the recently acquired knowledge gained from using crosslinked polyethylene and transferring it for use with various grades of polyethylene, previously believed not to be suitable for plumbing applications if not in the crosslinked state. However, it has been recognized that for some applications, the desirable flexibility of polyethylene, particularly medium density polyethylene (MDPE) is highly effective for use with plumbing applications which are not subject to either intense pressure situations or exposed to high temperatures. In yet another variation of this invention, the MDPE is a blend of both high density polyethylene (HDPE) and low density polyethylene (LDPE) to achieve the properties of MDPE.
In accordance with the present invention, there is provided a method for processing polyethylene polymers which will enable a work piece to have a one-piece formed/shaped ends from a single piece of tubing.
It is an object of this invention to provide a process for belling a tube of initial internal diameter to a larger internal diameter while maintaining at least a constant wall thickness throughout the tubing to produce a belled end.
It is another object of this invention to provide a process for forming a sealing surface shaped end on a tube wherein the shaping results in a thermally bonded end of constant diameter, the part having been formed from a polyethylene tube which achieves the physical properties of MDPE.